Fiber-optic disturbance detection using combined Michelson and Mach-Zehnder interferometers

ABSTRACT

A fiber-optic sensor can have a Michelson sensor portion and a Mach-Zehnder sensor portion. A first splitter-coupler can be configured to split incoming light between a first fiber portion and a second fiber portion. A first polarization-phase conjugation device can be configured to conjugate a polarization phase of incident light corresponding to the first fiber portion, and a second polarization-phase conjugation device can be configured to conjugate a polarization phase of incident light corresponding to the second fiber portion. Each of the first and second polarization-phase conjugation devices can be configured to reflect light toward a detector and through the respective first and second fiber portions. A coupler can be configured to join light in the first fiber portion with light in the second fiber portion, and a third fiber portion can be configured to receive light from the coupler and to illuminate a second detector.

This application is a U.S. National Phase application under 35 U.S.C.§371 of International Application Serial Number PCT/US11/052608 filedSep. 21, 2011, which also claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/393,298 and U.S. Provisional Patent ApplicationSer. No. 61/393,321, both filed Oct. 14, 2010, the contents of which arehereby incorporated by reference as if recited in full herein for allpurposes.

BACKGROUND

The innovations disclosed herein pertain to interferometer systems, andmore particularly, but not exclusively, to fiber-optic interferometersystems, such as, for example, systems used in security, surveillance ormonitoring applications. Some disclosed interferometer systems relate todetecting and locating disturbances (e.g., a disturbance to a secureperimeter, such as a “cut” on a fence, a leak from a pipeline, a changein structural integrity of a building, a disturbance to a communicationline, a change in operation of a conveyor belt, an impact on a surfaceor acoustical noise, among others) with one or more passive sensors.

Earlier attempts at using interferometer-based systems to detectdisturbances have met with varying degrees of success. For example, aMach-Zehnder interferometer can detect a phase-shift between two beamsof light split from a single collimated beam. When two respectiveoptical path lengths differ, the respective beams typically will be outof phase and a Mach-Zehnder interferometer can detect such a phasedifference. Thus, a Mach-Zehnder interferometer can detect a change inrelative optical-path lengths, such as can occur when one of a pair ofoptical conduits carrying optical signals is perturbed differently thanthe other of the pair. Nonetheless, a Mach-Zehnder interferometer alonecannot provide the location of such a disturbance or the magnitude ofthe difference in path lengths.

Systems including interferometers configured to detect a disturbancehave been proposed. For example, U.S. Pat. No. 6,778,717 discloses amethod that includes launching light in opposite directions through asingle Mach-Zehnder interferometer to form counter-propagating opticalsignals that can be modified by a perturbation of the interferometer(also referred to as a “disturbance” or an “event”). The '717 patentdiscloses that the position of such an event can be determined bysubstantially continuously and simultaneously monitoring respectivemodified counter-propagating optical signals and determining the timedifference between the separately detected modified signals. Thedisclosure in the '717 patent is incorporated herein in its entirety byreference.

U.S. Pat. Nos. 7,499,176 and 7,499,177 disclose improvements to thetechnology disclosed in U.S. Pat. No. 6,778,717. The '176 and '177patents are directed to methods and apparatus for actively controllingpolarization states of counter-propagating optical signals passingthrough a Mach-Zehnder interferometer so as to match phase and/oramplitude between the counter-propagating signals. With the technologydisclosed in U.S. Pat. No. 6,778,717, substantially matched polarizationstates are required to correlate the output corresponding to each of thecounter-propagating signals to the other respective signal outputs. Suchan interferometer is shown schematically in FIG. 1. The disclosure inthe '176 and '177 patents are incorporated herein in their entirety byreference.

To actively control the polarization states of the counter-propagatingsignals, a polarization controller is needed at each input of theMach-Zehnder interferometer's light paths. Such polarization controllersthat provide matched polarization states are costly. Also, since atleast some polarization controllers are configured to tune polarizationstates so that observed output signals have no amplitude- or phase-shiftbetween them, when a sensor is momentarily perturbed and apolarization-induced phase-shift between counter-propagating signals isthereby introduced, a significant amount of time can elapse after theperturbation and before the polarization controllers have suitablymatched polarization states to detect a subsequent perturbation.Therefore, a significant amount of time can elapse before a subsequentdisturbance can be detected and located accurately.

Therefore, systems as disclosed in U.S. Pat. Nos. 7,499,176, 7,499,177and 6,778,717 suffer serious deficiencies. For example, perimetersecurity systems incorporating such systems can be bypassed byintroducing a diversionary disturbance at one location and subsequentlycrossing a monitored perimeter at another location some distance awayfrom the location of the diversionary disturbance while the polarizationcontrollers are being “reset” (e.g., are attempting to re-matchpolarization states).

Other approaches for detecting disturbances have also been proposed. Forexample, U.S. Pat. No. 7,514,670, describes a low-cost system having adistributed plurality of sensitive “zones.” In particular, the '670patent discloses a system having an optical conduit configured to conveylight past a plurality of sensitive regions and to split off a fractionof light into each of the sensitive regions. Each of the sensitiveregions comprises, for example, an interferometer configured to detect adisturbance.

Since a portion of an incoming beam of light is diverted into eachsensitive region (or zone), such a system has practical limitations onthe number of zones that are possible when using a given light source.As a result of being limited to a particular number of zones, there isalso a practical limitation on the length of a perimeter that can bemonitored with such a system.

The '670 patent discloses that the presence of a disturbance can beisolated to a particular zone, so such a system can generally identifythe location of a disturbance. However, such a zone can span arelatively large distance, which might not provide a desired spatialresolution for many security applications. For example, some securityapplications require that a system identify the location of adisturbance to within several (e.g., less than about ten) meters (suchas, for example, to within between about 3 meters and about 5 meters).

Thus, a need remains for simpler and less costly systems for accuratelydetecting the existence, position or magnitude of a disturbance. Therealso remains the need for systems that provide these advantages over adistance of many kilometers. There also remains the need for systemsthat can detect the existence, position or magnitude of a subsequentdisturbance within fewer than about 3 seconds of an initial event ordisturbance.

SUMMARY

Innovative interferometer systems that overcome one or more of theforegoing or other needs are described. Some embodiments of suchinnovative systems comprise an apparatus configured to detect adisturbance (sometimes referred to as an “event” or a “target”) to anoptical conduit. In some instances, the presence of a disturbance,together with a position of the disturbance, can be detected. Someinnovative systems comprise a method for detecting such a disturbanceand its position. With some embodiments of such innovative systems, amagnitude of such a disturbance can also be determined. For example,some disclosed embodiments of optical (e.g., fiber optic) sensor systemsprovide one or more of the following advantages over distances up to andeven more than about 50 km away from active circuitry using passivelyterminated fiber optic sensors:

(1) detecting the presence of a disturbance;

(2) detecting a position of the disturbance; and

(3) detecting a magnitude of the disturbance.

Some innovative systems can provide these and other advantages overdistances up to, for example, about 65 kilometers (km) with one passivesensor, and up to, for example, about 130 km with first and secondpassive sensors extending in opposite directions.

These and other previously unattainable advantages are made possible, atleast in part, by an innovative interferometer-based sensorincorporating aspects of a Michelson sensor with aspects of aMach-Zehnder sensor.

In some innovative systems, the Michelson sensor portion includes afirst fiber portion and a second fiber portion. A first splitter-couplercan be configured to split incoming light between the first fiberportion and the second fiber portion. A first polarization-phaseconjugation device can be configured to conjugate a polarization phaseof incident light corresponding to the first fiber portion, and a secondpolarization-phase conjugation device can be configured to conjugate apolarization phase of incident light corresponding to the second fiberportion. Each of the first and second polarization-phase conjugationdevices can be configured to reflect light toward a detector (sometimesreferred to as a “Michelson detector”) and through the respective firstand second fiber portions. The Michelson detector can be positionedadjacent respective proximal ends of the first and the second conduits,and the respective polarization-phase conjugation devices can bepositioned adjacent the respective distal ends of the first and thesecond conduits.

In some innovative systems, the Mach-Zehnder sensor portion includes thefirst fiber portion and the second fiber portion, and the firstsplitter-coupler configured to split incoming light between the firstfiber portion and the second fiber portion. A coupler can be configuredto join a portion of light in the first fiber portion with a portion oflight in the second fiber portion, and a third fiber portion can beconfigured to receive light from the coupler and to illuminate a seconddetector (sometimes referred to as a “Mach-Zehnder detector”). Lightthat passes through the third fiber portion can illuminate the seconddetector independently of light reflected by the first or secondpolarization-phase conjugation devices.

In some instances, innovative interferometer systems also include apolarization scrambler configured to alter a polarization state of lightentering the first and second fiber sensor portions. The scrambler canintermittently (e.g., selectively, periodically or aperiodically) alterthe polarization so as to maintain a suitable signal-to-noise ratio atthe Mach-Zehnder detector (e.g., through the Mach-Zehnder sensor portionof the innovative interferometer).

The first fiber portion and the second fiber portion can extendlongitudinally of one passively terminated fiber optic cable. A proximalend of the fiber optic cable can be configured to couple to the firstdetector such that the Michelson sensor portion can illuminate the firstdetector. The proximal end of the fiber optic cable can be configured tocouple to the second detector such that the Mach-Zehnder sensor portioncan illuminate the second detector. The operative coupling between theMichelson sensor portion and the Mach-Zehnder sensor portion can bepositioned adjacent a distal end of the fiber optic cable. Therespective polarization-phase conjugation devices can be positionedadjacent a distal end of the fiber optic cable. Such a passivelyterminated fiber optic cable can extend up to about 65 km away from thefirst and second detectors, such as, for example, between about 40 kmand about 65 km away from the detectors. In other instances, a passivelyterminated fiber optic cable can extend between about 1 km and about 10km away from the first and second detectors, for example. Otherdistances, such as, for example, between about 10 km and about 20 km,between about 20 km and about 30 km, and between about 30 km and about40 km are also possible.

Innovative methods of identifying the location of a disturbance aredisclosed. For example, light can be launched into aninterferometer-based sensor having a Michelson sensor portion, aMach-Zehnder sensor portion, and an operative coupling therebetween. Acombined first signal portion and second signal portion can be observedby the Michelson sensor portion. The first signal portion can beobserved by the Mach-Zehnder sensor portion. A position of thedisturbance can be determined from a comparison of the first signalportion to the second signal portion. For example, the first signalportion observed by the Mach-Zehnder sensor portion can be subtractedfrom the combined first signal portion and second signal portionobserved by the Michelson sensor portion.

A magnitude of the disturbance can be determined, at least in part, fromobserved first and second signal portions, observed phase-shifts betweenthe first signal portion and the second signal portion, or both. Forexample, a magnitude of the disturbance can be determined, in part, byunambiguously counting fringes, unambiguously integrating the observedphase-shift over a specified duration, or both (e.g., by averaging therespective magnitudes determined from unambiguously counting fringes andunambiguously integrating the phase changes).

Computer-readable media and computer-implementable methods are alsodisclosed. Such media can store, define or otherwise includecomputer-executable instructions for causing a computing environment toperform innovative methods as disclosed herein. Related computingenvironments are also disclosed and can be special purpose or generalpurpose computing environments.

The foregoing and other features and advantages will become moreapparent from the following detailed description, which proceeds withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings show aspects of the innovating systemsdisclosed herein, unless specifically identified as showing a knownfeature from the prior art.

FIG. 1 shows a schematic illustration of a commercially available MachZehnder interferometer configured to use counter-propagating opticalsignals having actively matched polarization states.

FIG. 2 shows aspects of an innovative interferometer of the typedisclosed herein.

FIG. 3 shows aspects of an innovative interferometer system of the typedisclosed herein.

FIG. 4 shows aspects of an innovative disturbance detector as disclosedherein.

FIG. 5 shows aspects of a digital processor configured for use with adisturbance detector as shown in FIG. 4.

FIG. 6 shows a plot of two time-varying components of a first phasoroutput from a working embodiment of an innovative interferometer of thetype disclosed herein.

FIG. 7 shows a plot of two time-varying components of a second phasoroutput from the working embodiment of an innovative interferometer ofthe type disclosed herein.

FIG. 8 shows a plot of a time-varying total phase shift for each of thefirst and the second phasor outputs.

FIG. 9 shows a block diagram of a computing environment as disclosedherein.

FIG. 10 shows aspects of an alternative system configured to use a firstand second light sources and modulation separation to detect thepresence and location of a disturbance.

FIG. 11A shows a first phasor output plotted in polar coordinates. FIG.11B shows a second phasor output plotted in polar coordinates.

FIG. 12 is a table summarizing an innovative method as disclosed herein.

FIG. 13 is a table summarizing another innovative method as disclosedherein.

DETAILED DESCRIPTION

Various principles related to interferometer systems are describedherein by way of reference to exemplary systems. One or more of thedisclosed principles can be incorporated in various systemconfigurations to achieve one or more interferometer systemcharacteristics. Systems relating to perimeter security applications aremerely examples of innovative interferometer systems and are describedherein to illustrate aspects of the various principles disclosed herein.Some embodiments of disclosed innovations may be equally applicable touse in many other applications, such as, for example, detecting a leakin a pipeline, detecting a failure in a structure, detecting adisturbance to a ground surface, detecting a change in operation of aconveyor, etc.

Overview of Innovative Interferometer Systems

Interferometer systems as disclosed herein can detect a disturbance to asensor portion by comparing a phase shift between observed first andsecond optical signals that have travelled through a first (e.g., a“reference”) optical conduit and a second (e.g., a “sensor”) opticalconduit.

For example, the innovative interferometer 100 shown in FIG. 2 has afirst splitter/coupler 110 configured to split modulated light(indicated by arrows 112 a, 112 b) into first and second opticalconduits 114 a, 114 b (e.g., fiber-optic fibers, such as, for example,single-mode fiber optic fibers). First and second terminalsplitters/couplers 116 a, 116 b positioned adjacent a distal (terminal)end 118 of the respective first and second conduits are configured todirect a portion of light in the conduits to either (1) another coupler120 (referred to herein as a “Mach-Zehnder coupler”) configured torecombine light from the first and second conduits 114 a, 114 b, or (2)respective first and second polarization-phase conjugation devices 122a, 122 b that are configured to conjugate a polarization phase ofincident light. An example of such a polarization-phase conjugationdevice 122 a, 122 b is a Faraday rotational mirror, as indicated in FIG.2. A suitable Faraday rotational mirror can be obtained from OFRMF1-1310-A.

A Michelson interferometer can detect a difference in length between afirst optical path and a second optical path, and thus a disturbance toone of a pair of optical paths. A Michelson interferometer splits acollimated beam of light into a pair of light beams that followrespective light paths (e.g., through an optical conduit, such as asingle-mode optical fiber). At a terminal end of the respective opticalpaths, each respective beam is reflected such that it passes through thesame respective light path a second time, albeit in an oppositedirection compared to the first time the beam passed through it. A phaseshift between the reflected pair of beams indicates that the respectiveoptical paths have different optical lengths. Thus, a Michelsoninterferometer can be used to detect a disturbance to a pair of lightpaths that causes a net change in relative optical path lengths. Like aMach-Zehnder interferometer, a Michelson interferometer alone cannotidentify the location or magnitude of such a disturbance.

When arranged as shown in FIG. 2, the first splitter/coupler 110, thepair of optical conduits 114 a, 114 b (e.g., fibers) and respectivefirst and second polarization-phase conjugation devices 122 a, 122 bform, at least in part, a Michelson interferometer portion. Therespective first and second polarization-phase conjugation devices 122a, 122 b, e.g., Faraday rotational minors in the embodiment shown inFIG. 2 can be configured to swap the fast and slow polarization axes ofreflected light. Accordingly, a polarization state of each respectivereflected beam of light returning from the respective devices 122 a, 122b to the first splitter/coupler 110 through each of the conduits 114 a,114 b can be conjugate to the polarization state of the light as ittraveled from the splitter 110 to the devices 122 a, 122 b. The firstsplitter/coupler 110 can be configured to combine the respective beamsof reflected light travelling in the respective conduits 114 a, 114 band to divert a portion of the combined light to illuminate a firstdetector 124 (referred to herein as a “Michelson detector”). In someembodiments, the first detector is configured as a singlemode fiberpigtailed InGaAs photodiode.

Respective portions of light in the first and second conduits 114 a, 114b that are directed to the Mach-Zehnder coupler 120 can be recombined bythe coupler and directed into a return conduit 126 (e.g., a thirdsingle-mode fiber) optically coupled to a second detector 128 (referredto herein as the “Mach-Zehnder detector”) such that an optical signal inthe return conduit can illuminate the second detector. In someembodiments, the coupler 120 is a singlemode 3 dB fused coupler. As withthe Michelson detector 124, the Mach-Zehnder detector can be configuredas a singlemode fiber pigtailed InGaAs photodiode.

The Mach-Zehnder detector 128 can be positioned adjacent a modulatedlight source (identified in FIG. 2 as “modulation control”), theMichelson detector 124, or both. Such a configuration can provide acompletely passive optical sensor portion 130 extending away from anactive portion 132 a by as much as 50 km, or farther. As shown in FIGS.2 and 3, the active portions 132 a, 132 b comprise, respectively, amodulated light source, detectors 124, 128, and a demodulator. Theactive portion 132 b also includes a polarization scrambler. Such alight source can be a narrow line-width, singlemode pigtailed fiberlaser or other laser device having a narrow line-width. A suitablemodulator is an Agiltron NOPS-115111331 device. The passive portions 130comprise optical conduits 114 a, 114 b, 126, polarization-phaseconjugation devices 122 a, 122 b and the splitters/couplers 116 a, 116b, 120 positioned adjacent the distal end 118.

With such an interferometer 100, a signal detected at the Michelsondetector 124 can include a signal portion corresponding to the outboundlight mixed and confounded with a signal portion corresponding to theinbound light reflected by the polarization-phase conjugation devices122 a, 122 b. A signal detected at the Mach-Zehnder detector 128includes a signal portion corresponding to the outbound light. (In someinstances, one or more signal portions observed by the Michelson orMach-Zehnder detectors correspond to higher-order harmonics arising fromreflections. Such harmonics can be suitably filtered with low-passfilters.)

Signals from the respective detectors 124, 128 can be provided to ademodulator, and the demodulator can provide respective phasor outputs Iand Q (described below) to, for example, a post-processing apparatus,such as a computing environment described below. When the phasor outputsexceed a given threshold (e.g., detect a disturbance), thepost-processing apparatus can, for example, provide an alarm indicatingthe presence of a disturbance.

For example, if one of the optical conduits 114 a, 114 b is disturbed ata point 101, outbound light from the splitter/coupler 110 can reach thepoint 101 after a first time, T1, and a reflected portion of the lightcan reach the point 101 again at a second time, (T1+T2+T2). In such aninstance, the optical signal sensed at the Michelson detector 124 caninclude a first signal portion arising from a perturbation of theoutbound light mixed and confounded with a second signal portion arisingfrom a perturbation of the inbound (reflected) light. Since the mixedand confounded first and second portions (the “Michelson signalcomponents”) are observed with a single detector 124, they cannot beobserved independently. Stated differently, the two Michelson signalcomponents (outbound and inbound) cannot be separated out from theconfounded mixture.

In contrast, a signal observed at the Mach-Zehnder detector 128 cancontain a signal portion arising from a perturbation of the outboundlight only. Thus, in some disclosed approaches, the signal from theMach-Zehnder detector can be subtracted from the signal observed at theMichelson detector (a confounded mixture of the first, outbound, signalportion and the second, inbound signal portion) to obtain the secondsignal portion. Such a second signal portion can thus be used to extractthe Michelson first signal portion and this can be compared to the firstsignal portion observed by the Mach-Zehnder detector. A phase ortime-shift between the first signal portions of each interferometer canprovide a measure of the location of the disturbance, as described morefully below. Alternatively, or additionally, a comparison of the firstsignal portion and the second signal portion, once separated from theMichelson detector's signal, can provide a measure of the location of adisturbance. Likewise, being able to compare the first and secondseparated-out signal portions of the Michelson sensor can be utilized toprovide a measure of the location of the disturbance.

From the phase-shift, the delay T1 between the time light is launchedinto the sensor 130 and the time the light reaches a point ofdisturbance 101 and the delay, (T1+T2+T2), between the time the light islaunched into the sensor 130 and the time the light reflected by thepolarization-phase conjugation devices 122 a, 122 b reaches the point ofdisturbance 101 can be determined. With knowledge of the respectivedelays, the position of the point of disturbance 101 along the sensor130 can be calculated using disclosed methods. In addition, a magnitudeof the disturbance can also be determined using methods described below.

A signal amplitude at the Mach-Zehnder detector 128 can correspond, atleast in part, to a polarization state of light 112 entering the firstsplitter/coupler 110. As a polarization state of incoming light drifts,output from the Mach-Zehnder detector correspondingly changes. Forexample, under some polarization states, the output of the Mach-Zehndersensor can have an unsuitably low signal-to-noise ratio, which canresult in so-called “polarization fading” of the output from theMach-Zehnder detector. It was discovered that such polarization fadingcan be reduced or eliminated by randomly adjusting a polarization stateof incoming light to maintain a suitable signal-to-noise ratio, althoughthe exact polarization state of the incoming beam of light does notmatter. Accordingly, it was discovered that the polarization state canbe randomly varied, allowing the polarization state of the source toextend through a surface of the Poincare Sphere.

In FIG. 3, a sensor 100 a includes a polarization scrambler 132configured to intermittently adjust a polarization state of incominglight 112 to maintain a suitable signal-to-noise ratio at theMach-Zehnder detector 128. A polarization scrambler has little to noeffect on the Michelson portion of the sensor, since thepolarization-phase conjugation devices (e.g., Faraday rotation mirrors)conjugate the polarization state of reflected light, effectively undoingany effects that a change in polarization might have as light travelsbetween the splitter 110 and the devices 122 a, 122 b in the forwardpath and then from the devices back to the splitter/coupler and theMichelson detector 124.

A suitable polarization scrambler 132 is an electrically drivenpolarization controller-scrambler of the type produced by Agiltron,model NOPS-115111331. Such a device can be controlled by three or four(e.g., depending on the model) input voltages that can be varied over asuitable voltage range to provide myriad polarization states. In oneoperative embodiment, different drive signals can be applied to each(e.g., three or four) of the scrambler elements. Each respective drivesignal can be selected to allow a large number of polarization states tobe swept in a time-varying random manner.

Such random adjustment of polarization is quite different from theactive control and matched polarization states of independent beams oflight required in sensors of the type disclosed in U.S. Pat. Nos.7,499,176, 7,499,177 and 6,778,717. Such active control requires a verycomplex polarization controller scheme and is expensive to implement. Inaddition, the active polarization control of the prior art requires thesensor to intermittently pause while light having a new, suitablepolarization state is counter-propagated subsequent to a detecteddisturbance. In contrast, little to no delay is needed for the sensor100 a to detect a subsequent disturbance. Accordingly, a sensor 100, 100a can detect a disturbance subsequent to a first (e.g., “diversionary”)disturbance and cannot easily be by-passed using such a firstdisturbance to disrupt the sensor, overcoming a serious, long-feltdeficiency of the prior art.

Devices 132 capable of randomly changing the polarization state, as justdescribed, are substantially less expensive than polarizationcontrollers configured to match polarization states of different beamsof light, as required by systems disclosed in U.S. Pat. No. 6,778,717.In addition, randomly changing a polarization state of incoming light112 can occur much more quickly than matching the polarization states ofa pair of light beams to each other. Accordingly, hybridMichelson/Mach-Zehnder systems disclosed herein can more quickly respondto subsequent disturbances than systems that require matchedpolarization states of different beams of light and can be produced atlower cost than previously proposed sensors.

Paths of Light through Disclosed Interferometer Systems

Referring to FIGS. 2 and 3, the modulation control can emit ahighly-coherent beam of light 112 that enters the splitter/coupler 110that splits the light 112 into a first portion that travels through thefirst optical conduit 114 a and a second portion that travels throughthe second optical conduit 114 b. The light travelling through the firstoptical conduit 114 a enters an optical splitter 116 a positionedadjacent the distal end 118 of the first optical conduit, and the lighttravelling through the second optical conduit 114 b enters an opticalsplitter 116 b positioned adjacent the distal end of the second opticalconduit. The respective optical splitters 116 a, 116 b can split therespective beams of light into respective portions that enter the distalcoupler 120 and respective portions that impinge on the respectivepolarization-phase conjugation devices 122 a, 122 b.

The portions of light that impinge on the devices 122 a, 122 b arereflected, and each of the devices can conjugate a polarization state ofthe respective light portions. The reflected light portions can travelthrough the splitters 116 a, 116 b, into the respective conduits 114 a,114 b and back to the proximal splitter/coupler 110. The reflectedportions of light can be recombined at coupler 110 and a portion of therecombined light can illuminate the Michelson detector 124. Lightilluminating the Michelson Detector can define an optical signal that isinfluenced by each of the optical conduits 114 a, 114 b, including anydisturbances to the conduits. Light illuminating the Michelson Detectorhas passed through the pair of the conduits 114 a, 114 b twice—oncebefore illuminating the respective polarization-phase conjugationdevices and second time after being reflected therefrom and thus can beinfluenced twice by the same perturbation. The resulting optical signalcan provide twice the sensitivity compared to, for example, aMach-Zehnder or other interferometer in which an optical signal passesbut once through a disturbed optical path.

The respective portions that enter the distal coupler 120 can becombined, can pass through the return conduit 126, which imposes afurther optical delay (e.g., TL) that typically can equal T1+T2, and canilluminate the Mach-Zehnder detector 128. In contrast to lightilluminating the Michelson Detector 124, a major portion (e.g., exceptfor reflections and other “noise”) of light illuminating the MachZehnder Detector 128 has passed through the pair of the optical conduits114 a, 114 b once, and thus has been influenced by a given perturbationonce. In contrast, the optical received by the Michelson Detector 124has been influenced twice by the same disturbance, as described above.

Passive Optical Sensors

In some instances, the first and second optical conduits 114 a, 114 bcan have similar optical properties and similar lengths; in suchinstances it does not matter which of the conduits is considered thesensing conduit and the reference conduit. In some embodiments, thereference and sensor optical conduits 114 a, 114 b are physicallyseparate conduits positioned adjacent each other in a “bundle” (alsoreferred to as a “cable”). In other embodiments, the optical conduits114 a, 114 b are in physically separate cables.

For example, a conventional fiber optic bundle can include severalindividual optical fibers (e.g., single-mode fibers) shrouded by one ormore outer sheaths. One of the individual optical fibers can define thesensor conduit (e.g., 114 a) and another of the individual opticalfibers can define the reference conduit (e.g., 114 b). Yet another ofthe individual optical fibers can define the return conduit 126. Allfibers defining the conduits 114 a, 114 b, 126 can be positioned withinand shrouded by the common outer sheath(s). Although such optical fibersare positioned relatively close to each other (e.g., within severalmillimeters, of each other), a physical disturbance (e.g., an impact orperturbation) applied to the outer sheath(s) will be transmitted to eachof the individual fibers slightly differently. Moreover, each of theindividual fibers can respond (e.g., deform or have their respectiverefractive indices altered momentarily) to identical loads somewhatdifferently. Thus, in practice, a disturbance to the cable generallywill perturb the reference and the signal conduits 114 a, 114 bdifferently.

Since physical responses typically differ between the “sensor” conduitand the “reference” conduit, light travelling through the “sensor”conduit can arrive at a terminal end 118 of the sensor conduit at aslightly different time, and possibly with a different polarizationstate, than light travelling through the “reference” conduit. Thus,optical signals observed at each respective terminal end will usually beout of phase from each other by a nominal amount. When either or both ofthe sensor and reference conduits has been disturbed, the relative phaseof the optical signals observed at each respective terminal end willtend to shift from the nominal level from the undisturbed conduits. Inthe case of the Michelson interferometer, having the ability to separateand compare the delay between receiving the first (outbound) of theoptical signals and the second (inbound) of the optical signals (e.g.,an observed time-shift between the signals), and accounting forcharacteristics of the interferometer components (e.g., lengths ofoptical conduits, speed of light through the conduits), the position ofa disturbance can be determined. In another approach, the first signalportion observed by the Mach-Zehnder detector can be compared to theextracted first signal portion observed by the Michelson interferometerto provide a measure of the location of the disturbance.

Although many factors can cause an observed phase shift between signalsconveyed through the first and second optical conduits, a nominal, orbaseline, phase shift between observed signals of undisturbed referenceand sensor conduits can be determined. Thus, one can infer that a sensorcable (e.g., a bundle having a sensor conduit and a reference conduit)has been disturbed when a sufficiently large (or a threshold) deviationfrom a baseline phase shift is observed. In addition, observing such aphase-shift at more than one location in the total optical path (e.g.,outbound and inbound signals), combined with characteristics of thesensor cable (e.g., its length, the speed at which light travels througheach of the optical conduits), a location of the disturbance can beinferred.

In some embodiments, the third, insensitive conduit 126, which imposes afurther signal time delay of TL for the Mach Zehnder interferometer, canbe positioned adjacent one or both of the sensor conduit (e.g., conduit114 a) and the reference conduit (e.g., conduit 114 b). For example, anoptical cable can have a plurality of optical conduits within a commonsheath(s), as described above. One of the optical conduits can form theinsensitive conduit 126 configured to return light to the Mach-Zehnderdetector 128, and the other two conduits can form the sensor conduit andthe reference conduit, respectively. In such an embodiment, thereference and sensor conduits can be passively terminated adjacent adistal end 118 of the optical cable, as shown in FIGS. 2 and 3.

For example, respective splitters 116 a, 116 b and polarization-phaseconjugation devices 122 a, 122 b, and the distal coupler 120 can bepositioned adjacent a distal end 118 of the optical cable 130. Such aconfiguration can provide an entirely passive sensor to extend for adistance of up to, for example, about 65 km, from active components(e.g., the light source, the Michelson detector, the Mach-Zehnderdetector, a computing environment, etc.).

In some embodiments (described more fully below), a second passivesensor can extend in an opposite direction from a first passive sensorfor a distance of up to, for example, about 65 km. In such embodiments,a sensor system can extend up to, for example, about 130 km, with theactive components 132 a, 132 b being positioned at about a midpoint ofthe sensor system.

Multiplexed Phase Generated Carrier with Homodyne Demodulation

Modulation of current to a laser (e.g., a diode laser) can affect bothamplitude and wavelength (optical frequency) of emitted light. Eithereffect (amplitude or wavelength) can be used to drive a sensor asdisclosed herein. For example, if the sensor conduit and referenceconduit are substantially identical, amplitude modulation effects candominate the sensor's response. On the other hand, if one of theconduits is shorter than the other by, for example, a few meters, thenfrequency modulation effects may dominate the sensor's response. Sensorsdescribed herein can use either approach.

For simplicity and brevity, frequency modulation effects (and thusmethods relating to interferometers with different-length sensor andreference conduits) are fully described. Nonetheless, those of ordinaryskill will appreciate similar methods for obtaining disturbance andposition information using interferometers with equal-length sensor andreference conduits, and the corresponding amplitude modulation. Thefollowing methods can be implemented in a computing environment, asdisclosed more fully below.

Examples of Innovative Methods Related to Detecting a Disturbance andits Location

A Michelson sensor portion can provide an optical signal having twoinformation components. One component (I) can contain informationobtained from a disturbance as light travels from a light source(proximal end) to a polarization-phase conjugation devices (distal end).A second component (II) can contain information obtained as the lightpropagates back from the distal end to the proximal end. The MachZehnder sensor portion can provide an optical signal having oneinformation component, namely information obtained from the disturbanceas light travels from the source (proximal end) to the coupler (distalend). As an approximation, the response of the Mach Zehnder sensorportion can be assumed to be identical to the first component (I) of theMichelson sensor portion's response, particularly if they share the sameoptical conduits, and can be used in a transformation to isolate thesecond component (II) from the Michelson Interferometer response. A timedelay between the first (I) and second (II) responses can provide ameasure of the location of the disturbance along the length of thesensor. Such a transformation is now described.

The Michelson sensor portion's response (MI) can be described by aphasor, e.g., its in-phase (I) and quadrature (Q) response components.I _(MI)(t)=MI cos [φ(t−T2−TL)+φ(t+T2−TL)]  (1)Q _(MI)(t)=MI sin [φ(t−T2−TL)+φ(t+T2−TL)]  (2)where TL=T1+T2.The first (I) and second (II) components of the Michelson Interferometerare the first and second phase angle terms in each of Equations (1) and(2).

The Mach Zehnder sensor portion's response (MZ) can also be described byits phasor components, I and Q.I _(MZ)(t)=MZ cos [φ(t−T2)]  (3)Q _(MZ)(t)=MZ sin [φ(t−T2)]  (4)Since both the Michelson sensor portion and the Mach Zehnder sensorportion share the same sensor and reference conduits (e.g., conduits 114a, 114 b in FIGS. 2 and 3), each sensor portion “sees” the samedisturbance. Generally, the Mach Zehnder response is delayed because theMach-Zehnder sensor portion includes the insensitive conduit 126 thatreturns the Mach-Zehnder's optical signal to the Mach-Zehnder detector128. Mathematically, this delay can be represented as TL. Shifting theMichelson sensor portion's response in equations (1) and (2) by thecable delay TL, Equations (5) and (6) are arrived at:I _(MI)(t+TL)=MI cos [φ(t−T2)+φ(t+T2)]  (5)Q _(MI)(t+TL)=MI sin [φ(t−T2)+φ(t+T2)]  (6)The first (I) components of equations (5) and (6) have the same timingas the components of the Mach Zehnder response in equations (3) and (4),as expected since the outbound light travels the same path for the firstcomponent (I) in the Michelson sensor portion and the Mach-Zehndersensor portion. Equations (5) and (6) can then be rearranged usingtrigonometric identities to arrive at:I _(MI)(t+TL)=MI{cos [φ(t−T2)] cos [φ(t+T2)]−sin [φ(t−T2)] sin[φ(t+T2)]}  (7)Q _(MI)(t+TL)=MI{sin [φ(t−T2)] cos [φ(t+T2)]−cos [φ(t−T2)] sin[φ(t+T2)]}  (8)Substituting the Mach Zehnder responses shown in Equations (3) and (4)into equations (7) and (8), Equations (9) and (10) are obtained:

$\begin{matrix}{{I_{MI}\left( {t + {TL}} \right)} = {\frac{MI}{MZ}\left\{ {{{I_{MZ}(t)}{\cos\left\lbrack {\phi\left( {t + {T\; 2}} \right)} \right\rbrack}} - {{Q_{MZ}(t)}{\sin\left\lbrack {\phi\left( {t + {T\; 2}} \right)} \right\rbrack}}} \right\}}} & (9) \\{{Q_{MI}\left( {t + {TL}} \right)} = {\frac{MI}{MZ}\left\{ {{{Q_{MZ}(t)}{\cos\left\lbrack {\phi\left( {t + {T\; 2}} \right)} \right\rbrack}} + {{I_{MZ}(t)}{\sin\left\lbrack {\phi\left( {t + {T\; 2}} \right)} \right\rbrack}}} \right\}}} & (10)\end{matrix}$An “X” response can be defined as

$\begin{matrix}{{I_{X}(t)} = {\frac{MI}{MZ}{\cos\left\lbrack {\phi\left( {t + {T\; 2}} \right)} \right\rbrack}}} & (11) \\{{Q_{X}(t)} = {\frac{MI}{MZ}{\sin\left\lbrack {\phi\left( {t + {T\; 2}} \right)} \right\rbrack}}} & (12)\end{matrix}$Substituting equations (11) and (12) into (9) and (10), equations (13)and (14) are obtained:I _(MI)(t+TL)=I _(MZ)(t)I _(X)(t)−Q _(MZ)(t)Q _(X)(t)  (13)Q _(MI)(t+TL)=Q _(MZ)(t)I _(X)(t)+I _(MZ)(t)Q _(X)(t)  (14)Solving equations (13) and (14) for I_(X)(t) and Q_(X)(t), equations(15) and (16) are obtained

$\begin{matrix}{{I_{X}(t)} = {\frac{1}{{MZ}^{2}}\left\{ {{{I_{MZ}(t)}{I_{MI}\left( {t + {TL}} \right)}} + {{Q_{MZ}(t)}{Q_{MI}\left( {t + {TL}} \right)}}} \right\}}} & (15) \\{{Q_{X}(t)} = {\frac{1}{{MZ}^{2}}\left\{ {{{I_{MZ}(t)}{Q_{MI}\left( {t + {TL}} \right)}} - {{Q_{MZ}(t)}{I_{MI}\left( {t + {TL}} \right)}}} \right\}}} & (16)\end{matrix}$The equations derived in the foregoing are based on continuous functionsof time. Nonetheless, the detector 124, 128 outputs and these equationscan be digitized and converted to the sample data equivalents usingknown approaches. Thus, the equations just described can be implementedin a digital computing environment. A digital embodiment 200 of thesystem 100 a shown in FIG. 3 is illustrated in FIG. 4.

Before comparing the respective Mach-Zehnder and Michelson signals, theMichelson data can be mathematically delayed by a fixed amountcorresponding to, for example, the sensor length (TL). Such anintroduced delay can be used to address the differences between theMach-Zehnder and Michelson interferometer configurations.

Sampling and Modulation

In one embodiment, a sample frequency of fs=10 MHz (sample period ofTs=0.1 microseconds) can be used. A relative speed of an optical signal(compared to the speed of light in a vacuum) of 68.13% corresponds to10.36 meters per sample period. Based on this sample rate, a 1 km sensorcan have a delay line with 97 taps. For computational ease, 100 taps perkm of sensor are assumed (e.g., a 20 km sensor would require a 2000 tapdelay line).

A proposed modulation frequency isfm=fs/8=1.25 MHz  (17)This can become the carrier frequency associated with an in-Phase (I)component and the carrier for the Quadrature (Q) component can be at thesecond harmonic:2fm=fs/4=2.5 MHz  (18)

With a sample frequency of 10 MHz, this can provide ample room for ananalog anti-alias filter after the detector and before the ADC. Arepresentative sampling and modulation timing diagram for each detectoris illustrated in FIG. 5.

A frequency fu can denote an upper frequency content of the response. Insome instances, fu can be about 700 kHz. In a working embodiment in alaboratory environment, the highest frequency observed to date with thelab system is about 400 kHz for a Michelson sensor portion and about 200kHz for a Mach Zehnder sensor portion. In many instances, the Michelsonsensor portion's response frequency is less than about 200 kHz.

The modulation drive signal can be advanced (e.g., by TL) from ademodulation signal to account for time delay arising from the length ofthe interferometer. A magnitude of the advance can be reduced by a delaybetween the Mach Zehnder coupler and the Mach Zehnder detector.

Sensor performance can correspond, in some instances, to the performanceof the analog anti-aliasing filter. For example, the modulation schemedescribed above can cause a detector output rich in harmonics.

Locating a Disturbance from Observed Phase- or Time-Shift

A working embodiment of a phase integration approach to determining theposition of a disturbance is described. Under this approach, a timedifference between output of the Michelson sensor portion and the MachZehnder sensor portion arising from physical deformation of the opticalconduits is determined. As indicated by the equations above, such aphase shift can correspond to the position of the disturbance along asensitive portion of a hybrid Michelson and Mach Zehnder sensor 100, 100a, as shown in FIGS. 2 and 3. Such an innovative approach overcomes manydeficiencies (and the concomitant performance limitations) of prior artsensors (e.g., coaxial cable, electric field or acoustic cable).

The phasor output I and Q responses shown in FIG. 6 for the Michelsonsensor portion and FIG. 7 for the Mach Zehnder sensor portion arerepresented by equations (1), (2), (3), and (4), respectively.

An integration of the incremental phase measurements for each of therespective interferometers, over a short period of time, such as, forexample, about 10-20 milliseconds, can provide a measure of a change inlength of the optical conduits 114 a, 114 b relative to each other overthe selected integration time for each of the interferometers. Sinceboth interferometers share the same optical conduits, the integratedincremental phase measurements for each of the respectiveinterferometers should be the same, but shifted in time in accordancewith the position of the disturbance. Results of such an integration areshown in FIG. 8, showing a similar integrated phase pattern between thetwo interferometers, as well as a time delay between the signals thatcan be used to determine the position of the disturbance.

In FIG. 8, the vertical axis represents phase angle in radians. Forexample, a response range of between about −24 to about +35 radians cancorrespond to about 9.4 cycles (fringes), corresponding to about 14.57micrometers change in relative length of the sensor conduit and thereference conduit, assuming a light source having a wavelength of 1,550nanometers is used in an example embodiment. In this example, the 9.4cycle response can occur during a period of about 0.25 milliseconds,representing an average frequency response of about 37.6 kHz. The shiftin time between the responses provides a measure of the location of theunderlying disturbance to the sensitive portion of the hybridinterferometer.

To determine the position of the disturbance location, one integratedphase return can be continually subtracted from the other integratedphase return with different time shifts, and a least squares fit of theresulting data can be computed. Such an approach can yield a generally“V” shaped correlation curve, with the inferred location of thedisturbance being positioned at the apex of the “V”.

In determining the position of a disturbance, one of the displacementprofiles can be subtracted from the other with the different delayparameters as outlined above to find a best fit. In one workingembodiment, the two responses were displaced by between about 0 to about2000 sample points. At a 10 MHz sample rate, the range bin accuracy wasapproximately 10 meters. This can be improved upon by interpolating thelocation of the apex of the “V”.

The equations presented above and in this section are summarized insample data (e.g., digitized) form in the following section. Thedigitized form can be used in computer implementable methods.

Determining a Disturbance Type from Observed Signals

An optical signal can carry information related to the type ofdisturbance that has occurred. In some instances, an observed opticalsignal (e.g., a signal with a phase-shift, a change in amplitude, orboth), can correspond to a given type of disturbance (e.g., a leak in apipeline, an act of digging underground, a cut fence). Knowing how anoptical signal from a sensor varies in response to differentdisturbances can allow a user to determine what type of disturbance hasoccurred based on, at least in part, comparing an observed signal toanother observed signal generated in response to a known disturbance.

For example, a given disturbance can excite a given environment in asubstantially identical manner each time the disturbance occurs, therebyresulting in a substantially identical perturbation to a given sensoreach time the disturbance occurs. When a sensor is perturbed in a givenmanner, it can physically respond (e.g., undergo a strain or otherdeformation) in a corresponding manner, and thereby modify an opticalsignal in a corresponding manner.

In some instances, a sensor can be calibrated against differentdisturbances by recording each observed optical signal arising from eachof a variety of different disturbances. For example, a “look up” tableof signals corresponding to each disturbance can be generated. Asubsequent observed optical signal (e.g., arising from an unknowndisturbance type) can be compared to each of the recorded observedoptical signals, and a corresponding disturbance type can be inferredwhen the observed optical signal suitably matches a previously recordedoptical signal.

As noted above, a sensor can be excited (and thus can respond)differently in one environment compared to another environment. In someinstances, a sensor calibration (e.g., generation of a “look-up” table)can be completed after the sensor has been installed.

Digitized Equations for Determining a Position of a Disturbance

The equations presented above are expressed in their continuous timeform. Nonetheless, the analysis presented above can be performeddigitally. Accordingly, the equations are presented here in digitizedform.

The observed (e.g., measured) parameters are the Michelson and MachZehnder phasors {I_(MIi), Q_(MIi)} and {I_(MZi), Q_(MZi)} taken atinstant “i”. From the measured parameters {I_(Xi),Q_(Xi)} can bedetermined using:I _(Xi) ={I _(MZi) I _(MI(i+M)) +Q _(MZi) Q _(MI(i+M))}  (19)Q _(Xi) ={I _(MZi) Q _(MI(i+M)) −Q _(MZi) I _(MI(i+M))}  (20)The indexing constant, M, relates to the number of samples correspondingto the time delay TL. Note: the MZ terms in equations (15) and (16) areignored since they cancel when computing the small angle tangentapproximation.

The Mach Zehnder response {I_(MZi),Q_(MZi)} can be used to compute theMach Zehnder incremental angle using

$\begin{matrix}{{\delta\phi}_{MZi} = \frac{{Q_{MZi}I_{{MZ}{({i - 1})}}} - {I_{MZi}Q_{{MZ}{({i - 1})}}}}{{I_{MZi}I_{{MZ}{({i - 1})}}} + {Q_{MZi}Q_{{MZ}{({i - 1})}}}}} & (21)\end{matrix}$and the derived “X” response {I_(Xi), Q_(Xi)} can be used to compute theincremental “X” angle using

$\begin{matrix}{{\delta\phi}_{XZi} = \frac{{Q_{Xi}I_{X{({i - 1})}}} - {I_{Xi}Q_{X{({i - 1})}}}}{{I_{Xi}I_{X{({i - 1})}}} + {Q_{Xi}Q_{X{({i - 1})}}}}} & (22)\end{matrix}$When a disturbance is detected (e.g., a threshold is exceeded), the twodisplacement profiles can be computedφ_(MZj)=δφ_(MZi)+φ_(MZ(j−1)) j=0,1,2 . . . N  (23)φ_(Xj)=δφ_(Xi)+φ_(X(j−1)) j=0,1,2 . . . N  (24)The resulting displacement profiles can be correlated in time todetermine the location of the disturbance.Computing Environments

FIG. 9 illustrates a generalized example of a suitable computingenvironment 1100 in which described methods, embodiments, techniques,and technologies may be implemented. The computing environment 1100 isnot intended to suggest any limitation as to scope of use orfunctionality of the technology, as the technology may be implemented indiverse general-purpose or special-purpose computing environments. Forexample, the disclosed technology may be implemented with other computersystem configurations, including hand held devices, multiprocessorsystems, microprocessor-based or programmable consumer electronics,network PCs, minicomputers, mainframe computers, and the like. Thedisclosed technology may also be practiced in distributed computingenvironments where tasks are performed by remote processing devices thatare linked through a communications network. In a distributed computingenvironment, program modules may be located in both local and remotememory storage devices.

With reference to FIG. 9, the computing environment 1100 includes atleast one central processing unit 1110 and memory 1120. In FIG. 9, thismost basic configuration 1130 is included within a dashed line. Thecentral processing unit 1110 executes computer-executable instructionsand may be a real or a virtual processor. In a multi-processing system,multiple processing units execute computer-executable instructions toincrease processing power and as such, multiple processors can berunning simultaneously. The memory 1120 may be volatile memory (e.g.,registers, cache, RAM), non-volatile memory (e.g., ROM, EEPROM, flashmemory, etc.), or some combination of the two. The memory 1120 storessoftware 1180 that can, for example, implement one or more of theinnovative technologies described herein. A computing environment mayhave additional features. For example, the computing environment 1100includes storage 1140, one or more input devices 1150, one or moreoutput devices 1160, and one or more communication connections 1170. Aninterconnection mechanism (not shown) such as a bus, a controller, or anetwork, interconnects the components of the computing environment 1100.Typically, operating system software (not shown) provides an operatingenvironment for other software executing in the computing environment1100, and coordinates activities of the components of the computingenvironment 1100.

The storage 1140 may be removable or non-removable, and includesmagnetic disks, magnetic tapes or cassettes, CD-ROMs, CD-RWs, DVDs, orany other medium which can be used to store information and which can beaccessed within the computing environment 1100. The storage 1140 storesinstructions for the software 1180, which can implement technologiesdescribed herein.

The input device(s) 1150 may be a touch input device, such as akeyboard, keypad, mouse, pen, or trackball, a voice input device, ascanning device, or another device, that provides input to the computingenvironment 1100. For audio, the input device(s) 1150 may be a soundcard or similar device that accepts audio input in analog or digitalform, or a CD-ROM reader that provides audio samples to the computingenvironment 1100. The output device(s) 1160 may be a display, printer,speaker, CD-writer, or another device that provides output from thecomputing environment 1100.

The communication connection(s) 1170 enable communication over acommunication medium (e.g., a connecting network) to another computingentity. The communication medium conveys information such ascomputer-executable instructions, compressed graphics information, orother data in a modulated data signal.

Computer-readable media are any available media that can be accessedwithin a computing environment 1100. By way of example, and notlimitation, with the computing environment 1100, computer-readable mediainclude memory 1120, storage 1140, communication media (not shown), andcombinations of any of the above.

Other Embodiments

Using the principles disclosed herein, those of ordinary skill willappreciate a wide variety of possible embodiments of interferometersystems, particularly those configured to detect a disturbance. Forexample, a disturbance can be detected from observing polarizationseparation, wavelength separation, or both, in addition to or instead ofmodulation separation.

Modulation Separation with a Plurality of Light Sources

Although innovative interferometer systems 100, 100 a, 200 comprising asingle light source have been described above, plural light sources canbe used in a system 500, as shown, for example, in FIG. 10. For example,respective amplitudes of first and second light sources (respectivelylabeled “Laser Source 1” and “Laser Source 2” in FIG. 10) can bemodulated at respective first and second frequencies. The output of eachdetector D1, D2 can be demodulated to derive the respectiveinterferometer responses. As shown in FIG. 10, such a system can includetwo polarization phase conjugation devices (labeled as “Faraday RotatingMirror” in FIG. 10) corresponding to each optical conduit of theinterferometer.

In such a system 500, the first and second modulation frequencies arepreferably well above a target response band of frequencies to achieve asuitable sensitivity to disturbances. For example, modulationfrequencies can range between about 50 kHz to 750 kHz. In this example,it is suggested to use modulation frequencies of 20 MHz and 30 MHz. Sucha selection of frequencies allows the respective responses to be clearlydistinguishable and separable. As indicated in FIG. 10, a mixer can beused to drop the respective responses to a baseband before filteringwith a suitable low pass filter.

In FIG. 10, Laser Source #1 can be modulated at a first frequency f1.The modulated output of Laser #1 can be coupled to the lead-in cable andthe Michelson Interferometer #1. The response from this interferometermeasured at the detector, D1, can be operatively coupled to theprocessor.

Laser Source #2 can be modulated at a second frequency f2. The modulatedoutput of Laser #2 can be coupled to the lead-in cable and the MichelsonInterferometer #2. The response from this interferometer measured at thedetector, D2, can be operatively coupled to the processor.

For example, the respective output of each detector, D1 and D2, can passthrough anti-alias filters to remove high frequency components such thatthe outputs of the Analog to Digital Converts (ADC #1 and #2) faithfullyrepresent the desired analog output of the detectors. The sampling rateof the ADC converters can be fs. The corner frequency of the anti-aliascan be less than about half the sample frequency to be consistent withthe Nyquist sampling criteria.

In processing the digital signal, the outputs of the detectors can passthrough band-pass filters about the modulating frequencies. Such filterscan remove unwanted terms (e.g., unwanted cross-products that can arisein mixing) before mixing. The bandwidth of each of the filters can betwice the upper response frequency (i.e.: 2fu). Such filters tend toremove the baseband components from the detection process, leaving onlythe modulation frequencies used during demodulation, e.g., ω₁ and ω₂terms, and double these frequencies.

The signals can be demodulated by multiplying with suitable modulationfrequencies (e.g., by multiplying by +1, and −1 at suitable modulationrates; f1, 2f1, f2 and 2f2). This translates the responses down tobaseband where they are band pass filtered to obtain the desired phasoroutputs [I₁(t), Q₁(t)] and [I₂(t), Q₂ (t)].

Interpreting the I and Q Responses

The response from the interferometer shown in FIG. 10 (or in FIGS. 2 and3) can appear as a rotating vector that traces out a circle, as shown inFIGS. 11(A) and 11(B).

A direction of rotation can be determined by the relative change inlength of the sensor conduit and the reference conduit. In general, theresponse can be expected to rotate one direction for a number of cyclesunder the influence of a disturbance and to rotate in an oppositedirection as the conduits return to their respective undisturbedcondition.

Since a response typically contains many different frequency components,the direction of rotation may appear constant, although its angularspeed can vary considerably with time. The resulting pattern isgenerally unique to each disturbance, but is generally observed to bethe same from both detectors since both detectors are receiving signalsfrom the same fibers responding to the same disturbance. Nonetheless,the detectors receive the optical signals at different times, since therespective path lengths differ for the two interferometers. Accordingly,the position of the disturbance target can be identified by correlatingresponse #1 with the response #2.

As noted above, each disturbance to the sensitive conduits creates aunique response in terms of the number of cycles.

The table shown in FIG. 12 identifies several acts that together form aninnovative method as disclosed herein. For example, referring to thesystems shown in FIGS. 2 and 3, and the table in FIG. 12, light can belaunched into a fiber 1201. The light can be split into a first outboundportion and a second outbound portion 1202. The first outbound portioncan be split into a first reflection portion and a corresponding firstcoupling portion 1203. The second outbound portion can be split into asecond reflection portion and a corresponding second coupling portion1204. The first reflection portion can be reflected with a firstpolarization-phase conjugation device and the second portion can bereflected with a second polarization-phase conjugation device 1205. Thefirst reflection portion and the second reflection portion can becombined 1206. The first coupling portion and the second couplingportion can be combined 1207.

The table shown in FIG. 13 identifies several acts that together formanother innovative method as disclosed herein. For example, referringthe systems shown in FIGS. 2 and 3, and the table shown in FIG. 13,light can be launched into a fiber-optic sensor comprising a Michelsonsensor portion, a Mach-Zehnder sensor portion, and an operative couplingtherebetween. A combined first signal portion and second signal portioncan be detected from the Michelson sensor portion 1301. The first signalportion can be detected from the Mach-Zehnder sensor portion 1202. Thelocation of a disturbance can be sensed based on, at least in part, acomparison of the first signal portion and the second signal portion1304.

Disclosed Principles are Not Limited to Described Embodiments

This disclosure makes reference to the accompanying drawings which forma part hereof, wherein like numerals designate like parts throughout.The drawings illustrate specific embodiments, but other embodiments maybe formed and structural changes may be made without departing from theintended scope of this disclosure. Directions and references (e.g., up,down, top, bottom, left, right, rearward, forward, etc.) may be used tofacilitate discussion of the drawings but are not intended to belimiting. For example, certain terms may be used such as “up,” “down,”,“upper,” “lower,” “horizontal,” “vertical,” “left,” “right,” and thelike. These terms are used, where applicable, to provide some clarity ofdescription when dealing with relative relationships, particularly withrespect to the illustrated embodiments. Such terms are not, however,intended to imply absolute relationships, positions, and/ororientations. For example, with respect to an object, an “upper” surfacecan become a “lower” surface simply by turning the object over.Nevertheless, it is still the same surface and the object remains thesame. As used herein, “and/or” means “and” as well as “and” and “or.”

Accordingly, this detailed description shall not be construed in alimiting sense, and following a review of this disclosure, those ofordinary skill in the art will appreciate the wide variety ofinterferometer systems that can be devised and constructed using thevarious concepts described herein. Moreover, those of ordinary skill inthe art will appreciate that the exemplary embodiments disclosed hereincan be adapted to various configurations without departing from thedisclosed concepts. Thus, in view of the many possible embodiments towhich the disclosed principles can be applied, it should be recognizedthat the above-described embodiments are only examples and should not betaken as limiting in scope. And, although detailed claims have not beenpresented here since claims are not a necessary component for aprovisional patent application, I reserve the right to claim as myinvention all that comes within the scope and spirit of the subjectmatter disclosed herein, including but not limited to all that comeswithin the scope and spirit of the following paragraphs.

The invention claimed is:
 1. A method of detecting a disturbance using apassively terminated fiber optic sensor, the method comprising:launching light into a fiber-optic sensor comprising (i) a first fiberportion and a second fiber portion defining a sensitive portion of thesensor, and a third fiber portion defining an insensitive portion of thesensor, each of the first, second and third fiber portions extendingbetween respective proximal and distal ends, (ii) a firstsplitter-coupler positioned adjacent the proximal ends of the firstfiber portion and the second fiber portion and configured to splitincoming light between the first fiber portion and the second fiberportion via a third fiber coupler, (iii) a first coupler and secondcoupler position adjacent the respective distal ends of the first fiberportion and the second fiber portion and configured to combine lightfrom the first fiber portion with light from the second fiber portionand to direct the combined light into the third fiber portion, and (iv)a first polarization-phase conjugation device configured to conjugate apolarization phase of incident light corresponding to the first fiberportion and a second polarization-phase conjugation device configured toconjugate a polarization phase of incident light corresponding to thesecond fiber portion; detecting with a first detector a combined firstsignal portion and a second signal portion from light reflected by thefirst polarization-phase conjugation device and light reflected by thesecond polarization-phase conjugation device; detecting with a seconddetector the combined first signal portion from light combined by thethird fiber coupler and directed into the third fiber portion; anddetermining the location of a disturbance based on, at least in part, acomparison of the first signal portion and the second signal portion. 2.The method of claim 1, wherein the act of determining the location of adisturbance comprises subtracting the first signal portion from thecombined first signal portion and second signal portion.
 3. The methodof claim 1, further comprising determining a magnitude of thedisturbance based in part on a phase shift detected by the firstdetector, the second detector, or both.
 4. The method of claim 3,wherein the act of determining a magnitude of the disturbance comprisesthe act of counting fringes.
 5. The method of claim 4, wherein the actof determining a magnitude of the disturbance further comprisesintegrating the phase shift and averaging a magnitude of the disturbancedetermined from the act of counting fringes with a magnitude of thedisturbance determined from integrating the phase shift.
 6. The methodof claim 3, wherein the act of determining a magnitude of thedisturbance comprises integrating the phase shift.
 7. A non-transitorycomputer-readable medium containing executable instructions that, whenexecuted, cause an apparatus to perform a method comprising: launchinglight into a fiber-optic sensor comprising (i) a first fiber portion anda second fiber portion defining a sensitive portion of the sensor, and athird fiber portion defining an insensitive portion of the sensor, eachof the first, second and third fiber portions extending betweenrespective proximal and distal ends, (ii) a first splitter-couplerpositioned adjacent the proximal ends of the first fiber portion and thesecond fiber portion and configured to split incoming light between thefirst fiber portion and the second fiber portion, (iii) a first couplerand a second coupler positioned adjacent the respective distal ends ofthe first fiber portion and the second fiber portion and configured tocombine light from the first fiber portion with light from the secondfiber portion and to direct the combined light into the third fiberportion via a third fiber coupler, and (iv) a first polarization-phaseconjugation device configured to conjugate a polarization phase ofincident light corresponding to the first fiber portion and a secondpolarization-phase conjugation device configured to conjugate apolarization phase of incident light corresponding to the second fiberportion; detecting with a first detector a combined first signal portionand a second signal portion from light reflected by the firstpolarization-phase conjugation device and light reflected by the secondpolarization-phase conjugation device; detecting with a second detectorthe combined first signal portion from light combined by the third fibercoupler and directed into the third fiber portion; and determining thelocation of a disturbance based on, at least in part, a comparison ofthe first signal portion and the second signal portion.
 8. Thenon-transitory computer-readable medium of claim 7, wherein the act ofdetermining the location of the disturbance comprises subtracting thefirst signal portion from the combined first signal portion and secondsignal portion.
 9. The non-transitory computer-readable medium of claim7, wherein the method further comprises determining a magnitude of thedisturbance based in part on a phase shift detected by the firstdetector, the second detector, or both.
 10. The non-transitorycomputer-readable medium of claim 9, wherein the act of determining amagnitude of the disturbance comprises the act of counting fringes. 11.The non-transitory computer-readable medium of claim 10, wherein the actof determining a magnitude of the disturbance further comprisesintegrating the phase shift and averaging a magnitude of the disturbancedetermined from the act of counting fringes with a magnitude of thedisturbance determined from integrating the phase shift.
 12. Thenon-transitory computer-readable medium of claim 9, wherein the act ofdetermining a magnitude of the disturbance comprises integrating thephase shift.
 13. A method of constructing a fiber-optic sensor, methodcomprising: providing a first fiber portion and a second fiber portiondefining a sensitive sensor portion; providing a third fiber portiondefining an insensitive sensor portion, wherein each of the first,second and third fiber portions extend between respective proximal anddistal ends; and operatively coupling a first splitter-coupler to theproximal ends of the first fiber portion and the second fiber portionsuch that the first splitter-coupler is configured to split incominglight between the first fiber portion and the second fiber portionoperatively coupling a first coupler and second coupler to therespective distal ends of the first fiber portion and the second fiberportion such that the coupler is configured to combine light from thefirst fiber portion with light from the second fiber portion and todirect the combined light into the third fiber portion via a third fibercoupler; providing a first polarization-phase conjugation deviceconfigured to conjugate a polarization phase of incident lightcorresponding to the first fiber portion and a second polarization-phaseconjugation device configured to conjugate a polarization phase ofincident light corresponding to the second fiber portion, such that thefiber-optic sensor is configured to provide a first signal portion fromlight combined by the third coupler and directed into the third fiberportion and the fiber-optic sensor is further configured to provide thefirst signal portion in combination with a second signal portion fromlight reflected by the first polarization-phase conjugation device andlight reflected by the second polarization-phase conjugation device,wherein the first signal and the combined first and second signals areused to estimate a location of a disturbance to the sensitive portion ofthe sensor.
 14. A monitored installation with a passively terminatedfiber optic sensor, the monitored installation comprising: a fiber-opticsensor to receive light, the fiber-optic sensor comprising (i) a firstfiber portion and a second fiber portion defining a sensitive portion ofthe sensor, and a third fiber portion defining an insensitive portion ofthe sensor, each of the first, second and third fiber portions extendingbetween respective proximal and distal ends, (ii) a firstsplitter-coupler positioned adjacent the proximal ends of the firstfiber portion and the second fiber portion and configured to splitincoming light between the first fiber portion and the second fiberportion, (iii) a first coupler and a second coupler positioned adjacentthe respective distal ends of the first fiber portion and the secondfiber portion and configured to combine light from the first fiberportion with light from the second fiber portion and to direct thecombined light into the third fiber portion via a third fiber coupler,and (iv) a first polarization-phase conjugation device configured toconjugate a polarization phase of incident light corresponding to thefirst fiber portion and a second polarization-phase conjugation deviceconfigured to conjugate a polarization phase of incident lightcorresponding to the second fiber portion; a first detector configuredto detect a combined first signal portion and a second signal portionfrom light reflected by the first polarization-phase conjugation deviceand light reflected by the second polarization-phase conjugation device;a second detector configured to detect the combined first signal portionfrom light combined by the third fiber coupler and directed into thethird fiber portion; and a central processing unit (CPU) configured todetermine the location of a disturbance based on, at least in part, acomparison of the first signal portion and the second signal portion.15. The installation of claim 14, wherein the monitor region comprisesone or more of a fence, a pipeline, a rail line, a communication line, aconveyor, a structure affixed to the Earth, and a perimeter.
 16. Theinstallation of claim 14, wherein the passively terminated sensitiveoptical sensor extends away from the active portion between about 1 kmand about 65 km.
 17. The installation of claim 14, further comprising asecond optical sensor optically coupled to the active portion andextending away from the active portion between about 1 km and about 65km.
 18. The installation of claim 17, wherein the optical sensors definea region extending between about 100 km and about 130 km over which adisturbance can be detected by one or both of the sensors.